1. Field of the Invention
The present invention relates to an optical recording medium for recording and reading information by means of a laser beam.
2. Description of the Background
Optical recording media for recording and reading information by means of a laser beam have rapidly become practical on account of the recent development of fundamental technology of semiconductor lasers, recording materials and film making processes, and also by virtue of their ability to record a large amount of information. For an optical recording medium to be able to record information, a phase change leading to an optical change should take place at the part struck with a laser beam. Bubble (vacant space) making, pitting (pit or cavity formation), and amorphous state-crystalline state transfer are some of the recording methods proposed so far.
For example, techniques for recording media utilizing bubble making are disclosed in Japanese Patent Application Kokai Nos. 65340/1982 and 127937/1981.
In the case of recording media utilizing pit or cavity formation (hereinafter simply referred to as "pitting"), principal ones comprise as main component a chalcogen such as Te, Se, S, etc., and the technique is disclosed, e.g., in Japanese Patent Application Kokai Nos. 71193/1983, 110634/1977, and 146691/1982.
However, since most of chalcogen elements are chemically unstable, there is doubt about the durability of the media utilizing such elements, and attempts have been made to improve it by incorporating various additives, providing a protective film, or the like.
Generally, in the case of recording media by pitting, there has been encountered a drawback of inevitable scattering or depositing of portions of recording film onto the circumferences of the cavities being formed, making it difficult to improve the carrier to noise ratio (CNR) of the signals being read.
On the other hand, recording media by bubble making as proposed in Japanese Patent Application No. 127937/1981, while avoiding the above-mentioned drawback, still has the drawback of having a low recording sensitivity. Further, a recording medium proposed in Japanese Patent Application Kokai No. 65340/1982 shows an improvement on efficiency in utilization of a laser beam by providing a beam-reflecting layer between the substrate plate and an organic intermediate layer. In practice, however, it requires strictly specifying the thicknesses of the energy (light) absorbing layer and the intermediate plastic layer in relation to the wavelength of laser beam to be used, thereby causing the manufacturing process to be complex and the production yield to decrease.
An optical recording medium which is made up of a plastic substrate having a surface of minutely roughened structure and a thin metal film formed on the substrate is disclosed in Japanese Patent Application Kokai No. 135643/1984 and U.S. Pat. No. 4,616,237. The optical recording medium is characterized by more efficient absorption of the laser beam for recording as compared to a recording medium made up of a plain-surface plastic substrate and a thin metal film formed thereon and is therefore capable of recording with a lower power, which results in simplified construction of the recording medium. However, with the recent reduction in burden on the laser beam for recording, and development of multifunctional drives, laser cards, etc., demands for a high-sensitivity optical recording medium having a high CNR and causing reduced jitter have increased.